Chapter 4 Threads

1. Chapter 4Threads • Threads: Resource ownership and execution

2. Processes and Threads • Processes have two characteristics: • Resource ownership – a process includes an address space to hold the process image and may be allocated control or ownership of resources • Scheduling/execution – process execution follows an execution path that may be interleaved with other processes • These two characteristics are treated independently by the operating system

3. Processes and Threads • The unit of dispatching is referred to as a thread or lightweight process • The unit of resource ownership is referred to as a process or task

4. Multithreading • The ability of an OS to support multiple, concurrent paths of execution within a single process.

5. Single Thread Approaches • MS-DOS supports a single user process and a single thread • Some UNIX, support multiple user processes but only support one thread per process

6. Multithreading • Java run-time environment is a single process with multiple threads • Multiple processes and threads are found in Windows, Solaris, and many modern versions of UNIX

7. Processes • In a multithreaded environment, a process is defined as • a unit of resource allocation: a virtual address space which holds the process image • a unit of protection: protected access to processors, other processes (for IPC), files, I/O resources

8. One or More Threads in Process • Each thread has • An execution state (running, ready, etc.) • Saved thread context when not running • An execution stack • Some per-thread static storage for local variables • Access to the memory and resources of its process (all threads of a process share this)

9. One view… • One way to view a thread is as an independent program counter operating within a process.

10. Threads vs. processes

11. Threads vs. processes • The thread control block contains register values, priority, and other thread-related state information. • All threads of a process share the state and resources of that process. • reside in the same address space • have access to the same data • when one thread alters a data, other threads see the results • when one thread opens a file, other threads can also access that file

12. Benefits of Threads • Takes less time to create a new thread than a process (can be 10 times faster) • Less time to terminate a thread than a process • Switching between two threads takes less time that switching processes • Threads can communicate with each other • without invoking the kernel

13. Benefits of Threads • If an application is implemented as a set of related units of execution, it is far more efficient to do so as a collection of threads rather than a collection of separate processes

14. Example Applications of Threads • In a file server, a new thread can be spawned for the file management program per each new file request  multiple threads within the same process can be executing simultaneously on different processors • In a spreadsheet program, one thread could read user input while another thread executes user commands • In a word processor, a thread can be created to do periodic backup asynchronously • In a multithreaded process, one thread can compute one batch of data while another thread reads the next batch from an I/O device

15. Threads vs. processes • Like processes, threads have execution states • Running, Ready, and Blocked • Some states are process-level • Suspend: if a process is swapped out, all of its threads are necessarily swapped out because they all share the address space of the process

16. Threads vs. processes • Like processes, thread may synchronize with one another. • Any alteration of a resource by one thread affects other threads in the same process • consider two threads each try to add an element to a linked list at the same time

17. Example: Remote Procedure Call • Consider: • A program that performs two remote procedure calls (RPCs) • to two different hosts • to obtain a combined result.

18. RPCUsing Single Thread The program has to wait for a response from each server in turn.

19. RPC Using One Thread per Server The program waits concurrently for the two replies.

20. Multithreading on a Uniprocessor Interleaving of multiple threads within multiple processes on a uniprocessor